It is desirable that air taken into a combustion chamber of an internal combustion engine is suitably swirled in order to promote mixing of air with fuel. This swirl is commonly provided by giving directivity to intake air by an intake port formed into a helical shape. When the load of the engine is light, a swirl is produced to promote mixing of intake air with fuel thereby improving the combustion efficiency. However, when the engine is heavy-loaded, and suction of a large quantity of intake air is required, a large resistance against intake air is caused by the helical shape of the intake port, resulting in a decrease in the charging efficiency. With a view to obviate such a drawback, a partition wall or a separate port is commonly provided in the intake port according to the prior art, and an associated valve is provided so as to change the magnitude of the swirl.
In the prior art device described above, the provision of the partition wall or the separate port in the intake port or the additional provision of the valve results in a complex structure of the intake port and is undesirable from the viewpoints of operation, manufacture and cost.
When the valve provided in the intake port is full closed, such a trouble has occurred in which the inclination angle of the valve relative to the centerline of the intake port becomes large thereby producing a large resistance against flow of intake air. To avoid such a trouble, an attempt is made in which the length of the valve from its root end to its front end is extended to decrease the inclination angle of the valve when closed. When such a valve is used, the resistance against intake air flow can be decreased to decrease the loss of energy. However, the use of the valve having such a large length gives rise to the defect that the shape of the intake port is subject to various design restrictions, and the force required for actuating the valve becomes also large resulting in a large loss of power for driving the valve, although the loss of energy of intake air flow is decreased. In order that the swirl can be produced regardless of the shape of the intake port, the valve may be arranged to be inserted into and withdrawn from the interior of the intake port. However, even when such a withdrawable valve is used, the defect attributable to the increased resistance against intake air flow is not still obviated although the shape of the intake port is now free from various design restrictions.